J. Phys. B: At. Mol. Opt. Phys. 32 (1999) 197–212. Printed in the UK PII: S0953-4075(99)96425-8 The capture of slow antiprotons in noble gases J S Briggs†§, P T Greenland‡ and E A Solov’ev† † Macedonian Academy of Sciences and Arts, Skopje, Macedonia ‡ Blackett Laboratory, Imperial College, University of London, London, UK Received 3 August 1998 Abstract. The capture of slow antiprotons (energies less than 1.0 au, i.e. roughly 27 eV) by the rare gas atoms helium, neon and argon is considered. Appropriate to this low velocity, the capture cross sections are calculated using the adiabatic ‘hidden-crossing’ theory in which the collision complex is viewed as a transient diatomic molecule with the positively-charged atomic ion and antiproton as nuclei. In addition to the total capture cross section, estimates of the percentage population of long-lived ‘circular’ states is given. Our calculations suggest that a few per cent of captured antiprotons occupy these states in helium but in the case of argon or neon the probability of primary capture into such long-lived states is negligible. These results are at variance with previous calculations of antiproton capture cross sections. 1. Introduction The stopping, capture and annihilation of antiprotons in liquids and gases has been much studied experimentally (Yamazaki et al 1989, 1993, Iwasaki et al 1991, Morita et al 1994, Widmann et al 1995, Hori et al 1998). One noteworthy feature of these experiments has been the observation that, although most stopped antiprotons annihilate promptly (in times of ∼10 −11 s), a few per cent can survive for times up to microseconds, if the stopping medium is solid, liquid or gaseous helium. In neon or argon, however, these long-lived states are not observed. Antiprotons stop in matter mostly by being captured by atoms. The initial translational kinetic energy of the antiproton is transferred to an atomic electron which is ejected from the atom so that the antiproton can take its place. Although this process can occur for any initial antiproton kinetic energy, the difficulty of transferring large amounts of energy and angular momentum via the Coulomb interaction implies that the capture process is most efficient when the antiproton slows down to energies corresponding to the ionization energy of the atom. Then ejection of an electron of any energy can proceed only by simultaneous capture of the antiproton. Again, however, minimum energy transfer, leading to an electron with zero energy in the continuum and an antiproton in a high-lying state, will be most favoured. In fact this is the process upon which we will focus attention in what follows. Theoretical and experimental studies suggest that the long-lived states in helium correspond to states of the He 2+ –e–¯ p system in which the electron is in its ground state and the antiproton is in a state with an orbital radius and binding energy roughly the same as the helium electron which it has replaced. In turn this implies that, in the simplest independent-particle picture, the antiproton has a principal § Permanent address: Theoretical Quantum Dynamics, Faculty of Physics, University of Freiburg, 79104 Freiburg, Germany. 0953-4075/99/020197+16$19.50 © 1999 IOP Publishing Ltd 197